The RefererIgnore directive adds to the list of strings to
ignore in Referer headers. If any of the strings in the list is
contained in the Referer header, then no referrer information
will be logged for the request. Example:

A program to receive the referrer log information on its
standard input. Note that a new program will not be started
for a VirtualHost if it inherits the RefererLog from the main
server.

Security: if a program is used, then it will
be run under the user who started httpd. This will be root if
the server was started by root; be sure that the program is
secure.

Security: See the security tips document
for details on why your security could be compromised if the
directory where logfiles are stored is writable by anyone other
than the user that starts the server.